Effective removal of heavy metals from wastewater of various origins remains a challenge for environmental protection technology. Current techniques for wastewater treatment involve large sedimentation pools, sand filters, large quantities of flocculants or other polymeric additives. A more efficient process that reduced equipment size and eliminated the use of flocculants would be a significant advancement in the art.
Several different methods have been previously suggested for achieving this goal.
Ion exchange using zeolite (M. J. Zamzow et al. Sep.Sci.Technology 25, 13-15 (1990) 1555-1569), quartz (T. W. Healy et al Adv. Chem.Ser 79 (1968)62) or alumina (M. Uberoi et al. Prep.Pap.Am.Chem.Soc.Div Fuel Chem 4 (1991)36) is unsuitable because these adsorbents work only for relatively high concentration of metal ions.
Direct electrochemical methods such as electrodialisis and other ion filtration techniques (Water Quality and Treatment, A Handbook of Community Water Supplies, Ed.:R. D. Letterman McGraw-Hill, 1999) are too expensive for large volumes of wastewater.
The most promising approach is to use silica as the heavy metals ion adsorbent. There are four US Patents issued (3 during the last 2 years) suggesting different silica based technologies for heavy metal removal: U.S. Pat. No. 5,370,827; 5,871,648; 5,904,853; and 6,077,439.
The ability of silica to adsorb heavy metals is well known and described in the following works: D. L. Dugger et al. J.Phys.Chem, 68 (1964) 757060; R. James and T. Healy, J.Coll. Interface Sci., 40 (1972) 65-81; K. Subramaniam et.al, Colloids and Surfaces 177 (2001) 133-146.
All four patents enumerated above suggest adding silica (or activated silica) directly to the wastewater. The silica particles, having adsorbed the offending metal ions, must then themselves be subsequently removed from the process stream Although in principle this can be achieved using filtration or sedimentation techniques, in practice the silica particles or aggregates used as the adsorbent must be large enough to facilitate their subsequent removal. In fact, the schemes suggested in all of these patents are suitable only for large silica particles, perhaps porous, having a surface area of less than 50-400 m2 per gram. On the other hand colloidal silicas are routinely produced using wet precipitating technology which makes it possible to synthesize very small particle with size on the nanometer scale with surface areas of greater than 200 m2 per gram. Ludox is a widely known colloidal silica manufactured initially by E. I. du Pont, now by Grace Davison. There are several grades with different sizes varying roughly from 10 to 30 nm. There are also many different colloidal silicas produced specifically for chemical-mechanical polishing of semiconductor wafers, which is considerably more expensive than Ludox silica. We know of only one instance where colloidal silica is suggested as an adsorbent: U.S. Pat. No. 4,272,523. This patent deals with a biological application, a method of making fibrinogen. It does not speak of the use of colloidal silica in relationship to metal ion extraction.
We suggest here a new removal scheme for taking advantage of the high affinity of the silica surface for heavy metals. The roots of this new scheme derive from U.S. Pat. No. 4,963,264 issued in 1990. This patent describes a so-called “selective dialysis with size selective membranes”.
Selective dialysis is one variation of a wide class of membrane technologies. The solution to be treated is separated from the adsorbent dispersion by a membrane having a pore size somewhat smaller than size of the adsorbent particles. The membrane thus prevents the adsorbent particles from passing into the treated water stream, but importantly does allow molecules and ions to diffuse from the treated water into the adsorbent dispersion where they can then be trapped by adsorption on the surface of these adsorbent particles. U.S. Pat. No. 4,963,264 suggest that the separating membrane be organized as a collection of the hollow fibers. The treated water can be either on the inside or outside of the fiber. The adsorbent dispersion is placed on the opposite side relative to the treated water. This patent does mentions various polymer adsorbents for selective dialysis but does not mention silica This patent also does not specify the direction flow for the treated water and adsorbent dispersions, which might be very important to optimize the process efficiency, as we will show below.
There are two important and novel features in the current invention as compared with U.S. Pat. No. 4,963,264. First, we describe the use of silica as the affinity adsorbent within a general framework of selective dialysis. Second, we propose using this selective dialysis in a counter-flow mode. These two features allow us to achieve the following system improvements.
We completely eliminate the need for removing the silica particles from the treated water after adsorption.
We can reach much higher utilization of the adsorption capacity. In the traditional scheme the utilization of the adsorption capacity corresponds to the equilibrium with the output water stream (stream after treatment) with the low effluent concentration in it. In the suggested counter-flow scheme the utilization corresponds to the initial concentration in the waste water stream which exceeds the output concentration by orders of magnitude. Consequently, we can achieve much higher adsorption and capacity utilization.
The new scheme allows us to use colloidal silica with very small size and high surface area, which was impossible in the traditional scheme when silica is mixed directly to the treated water. It is important because colloidal silica has several advantages over polymer adsorbents mentioned in the U.S. Pat. No. 4,963,264. Colloidal silica:                can be used at high volume fraction, up to 30%;        is much cheaper than polymer adsorbents;        is already produced in large quantities.        is stable with respect to aggregation over a wide pH range.        is negatively charged over a wide pH range.        spontaneous solidifies in contact with air.        
This last point is particularly important in view of the severely restricted discharge of untreated residuals under the National Pollutant Discharge Elimination System of the Clean water Act. Landfills continue to be the primary method of disposal of the solid waste and sludge. The buried refuse is subject to leaching by percolating water derived from rain and snowmelt. The liquid that is derived from these process is known as leachate. The ground water pollution by leachate became the serious ecological problem. This invention offers a unique solution to this problem. Adsorption of metal ions reduces the surface charge on the silica and it spontaneously become a solid glass. The heave metal ions are captured irreversible in this process. However, in order to eliminate this solidification within the device itself, we propose to continuously monitor the zeta potential of the colloidal silica using an electroacoustic zeta potential probe described in the U.S. Pat. No. 6109098.